Tl3 PSe4 Compound, single crystals, and acousto-optical devices

ABSTRACT

The compound Tl 3  PSe 4  is disclosed. Non-conducting single crystals of the compound are prepared which have outstanding acousto-optical properties including an exceptionally high acousto-optical figure of merit. The crystals are used in various acousto-optical devices including a display device, a laser modulator, a non-collinear acousto-optic filter, and an acoustic delay line.

CROSS-REFERENCE TO RELATED APPPLICATIONS

This application is related to application Ser. No. 242,986 filed Apr.11, 1972, entitled "Tl₃ AsS₄ Crystals and Acousto-Optical System," nowU.S. Pat. No. 3,799,659.

This application is related to application Ser. No. 392,695, filed Aug.29, 1973 by T. J. Isaacs et al. titled "Tl₃ VS₄ and Tl₃ NbS₄ Crystalsand Acousto-Optical Devices."

This application is related to application Ser. No. 540,192 filed Jan.10, 1975 by T. J. Isaacs et al. titled "Tl₃ TaS₄ and Tl₃ TaSe₄ Crystalsand Acousto-Optical Devices."

BACKGROUND OF THE INVENTION

In 1932 Brillouin discovered that high frequency sound waves can causediffraction of light. With the advent of the laser and advances in highfrequency acoustic techniques many applications for this phenomenon havebeen found such as display devices or laser modulators.

A sound wave moving in crystal is composed of alternating compressionand rarefaction fronts. The indices of refraction in these fronts aredifferent, so that the crystal acts as a diffraction grating,diffracting light which passes through it, the angle of diffractionincreasing as the frequency of the sound wave increases, and the amountof light diffracted increasing with the intensity of the sound wave.

There are two modes of diffraction, the Debye-Sears mode and the Braggmode. The Debye-Sears mode is obtained if the width of the acoustic beamis less than about A² /(4λ) and the Bragg mode is obtained if the widthof the acoustic beam is greater than about A² /(4λ) where A is theacoustic wavelength and λ is the light wavelength. In both modes theacoustic wavelength A must be greater than the light wavelength λ, and λmust be within the transparency region of the crystal. In theDebye-Sears mode light enters the crystal parallel to the acoustic wavefronts (0° diffracting angle) and is multiply-diffracted into manyimages or orders of the initial light beam. In the Bragg mode lightenters the crystal at the Bragg angle φ to the acoustic wave frontswhere sin φ=λ/Λ. In this mode the acoustic wavelength and the Braggangle are matched to the particular light wavelength, and a single imageis diffracted from the crystal at the Bragg angle φ to the acoustic wavefronts.

A good acousto-optical material should have a high figure of merit, M₂,a measurement of the amount of light diffracted for a given amount ofacoustic power, where M₂ =m⁶ p² /p v³ and n is a refractive index, p isthe photoelastic coefficient, p is the density, and v is the acousticvelocity. As the formula indicates, a low velocity will give a highfigure of merit and, in addition, it will give a greater delay per unitlength if the crystal is used in a delay line thus permitting acousticsignal processing devices to have smaller physical dimensions. A goodacousto-optical material should also have a low acoustic attenuation,allowing a high frequency wave to propagate a long distance before it isabsorbed.

The following table gives a few of the properties of the bestacousto-optical materials currently known for use in the rear infraredregion of the spectrum.

    ______________________________________                                                 Optical                                                                       Transmission                                                                              Acoustic Velocity                                                                           Fig.                                       Material Range (m)   (× 10.sup.5 cm/sec)                                                                   of Merit                                   ______________________________________                                        Ge         2-20      5.5           525                                        As.sub.2 S.sub.3 glass                                                                 0.9-11      2.6           230                                        GaAs       1-11      5.15           93                                        Tl.sub.3 AsS.sub.4                                                                     0.6-12      2.15          330                                        PbMoO.sub.4                                                                            0.4-5.5     3.83           24                                        ______________________________________                                    

PRIOR ART

An article entitled "Some Ternary Thallium Chalcogenides" by C.Crevecoeur appears in the January-June 1964 volume (No. 17) of ActaCrystallographica on page 757. That article describes the preparationand characteristics of the isomophous compounds Tl₃ VS₄, Tl₃ NbS₄, Tl₃TaS₄, Tl₃ VSe₄, Tl₃ NbSe₄, and Tl₃ TaSe₄.

SUMMARY OF THE INVENTION

We have discovered the existence of the compound Tl₃ PSe₄, and havefound that large single crystals can be grown from this compound. Wehave also found that crystals of Tl₃ PSe₄ have the highestacousto-optical figure of merit and lowest acoustic velocity of anyknown material, and that acousto-optical devices employing thesecrystals have outstanding properties. The crystals also have goodphysical properties such as resistance to moisture, they can be grown tolarge sizes, and they have low acoustic velocities and acousticattenuation.

DESCRIPTION OF THE INVENTION

FIG. 1 is an isometric diagrammatic drawing of a display device;

FIG. 2 is a diagrammatic drawing of a laser modulator of the internalconfiguration;

FIG. 3 is a diagrammatic drawing of a laser modulator of the externalconfiguration;

FIG. 4 is a diagrammatic drawing of an acoustic delay line;

FIG. 5 is a diagrammatic drawing of a non-collinear acousto-opticalfilter.

PREPARATION OF THE COMPOUND AND CRYSTAL

The compound of this invention, Tl₃ PSe₄, may be prepared by mixingtogether very pure stoichiometric quantities of the elements involvedand melting them together until they have reacted to form the compound.The compound and the resulting crystal may be made slightlynon-stoichiometric in order to relieve internal stresses. Up to about50% of the phosphorus may be substituted for with arsenic.

The crystal may be prepared by the Stockbarger technique in which thecompound is sealed in a quartz tube under argon, melted, and loweredvery slowly (10 to 15 mm/day) through a two-zone furnace having a steeptemperature gradient (8° to 12° C/mm) at the melting point of thecompound. The compound melts congruently at approximately 436° C±10° C.

THE CRYSTAL

The crystal of this invention is biaxial nonpiezoelectric, andorthorhombic. Its space group is Pcmm, its Laue class is mmm, and thediffraction aspect derived from X-ray data is Pc*n. The length of theaxes of the crystal are about a=9.270A, b=11.047A, c=9.059A, and itstransparency region is about 0.78 μ m to about 17 μ m, although strongabsorption peaks exist at 9.53 μ m, 9.76 μ m, from 11.4 μ mato 12.5 μ m,and from 15.6 μ m to 16.1 μ m.

The crystals should be as long as possible in order to maximize theoutput power, but if the crystal is too thick (i.e., more than about 10cm) light loss due to absorption will be high. On the other hand, thecrystal should not be too thin in the direction of light propagation asthis will result in poor interaction between the light and sound andtherefore a low intensity defraction, but a crystal as small as 1 mmlong can be optically useful. From the practical point of view oforienting and polishing faces on the crystal and attaching to it atransducer to generate acoustic waves, the crystal must have dimensionsof at least .1 mm. The width of the crystal should be at least as wideas the input beam can be focused, about 10⁻³ mm, so that the light isnot wasted. For acousto-optical applications, the crystal must be largeenough to produce a Bragg interaction between sound and light. Thatrequires at least 10 acoustic wave fronts, which means a minimum lengthabout 2 × 10⁻³ mm is required at an acoustic frequency of 300 MHz.Preferably the crystal should be at least about .05 cm in diameter andabout 1 cm long to have practical usefulness in most applications. Thecrystal also preferably has at least two polished parallel opticalfaces, which preferably are perpendicular to those axes of the crystalalong which sound propagates as a pure longitudinal or .[.skew.]..Iadd.shear .Iaddend.mode.

THE SOUND WAVES

The sound wave may be a longitudinal wave, where the particle motion isin the direction of propagation of the wave, or it may be a shear wave,where the particle motion is perpendicular to the propagation directionof the wave, or it may be a combination of both. Preferably, it iseither pure shear or pure longitudinal because the two waves travel atdifferent velocities and quickly become out of phase. For delay lineapplications shear waves are desirable because of their lower velocity.Pure shear waves are obtained by propagating the wave in a pure sheardirection (determined from the crystal symmetry) using a shear wavegenerating transducer such as a Y cut or A-C cut quartz, which is gluedto the crystal. Longitudinal waves are obtained by propagating the wavealong the c-axis or another pure longitudinal direction using alongitudinal wave generating transducer such as X-cut quartz which isglued to the crystal.

DISPLAY DEVICES

In display device a light beam is directed at the crystal and thedeflected beam which leaves the crystal is directed at some type ofviewing screen.

In FIG. 1 RF generators 1 and 2 send RF signals to transducers 3 and 4respectively which respectively generate vertically moving andhorizontally moving sound waves in crystal 5, preferably in the Braggmode so that there is only one diffracted beam. The light, which ispreferably parallel and polarized for good resolution, is obtained fromlaser 6 which generates a coherent beam of light 7 directed at one ofthe two parallel optical faces 8 of crystal 5. Light passing throughcrystal 5 is directed at various spots 9 on viewing screen 10 by meansof the vertically and horizontally moving sound waves generated bytransducers 3 and 4. Lens 11 focuses the light at the spot.

The illuminated spots may each be a page of information which is thenoptically enlarged and projected on a second viewing screen (not shown).The illuminated spots could also in themselves form a pattern. Forexample, viewing screen 10 could be an infrared-sensitive phosphorcoated screen such as zinc sulfide doped with lead and copper andflooded with UV light and the successive illumination of selected spotswould form a picture similar to a TV picture. Or, viewing screen 10could be infrared or thermally quenched UV-excited phosphor screen whereultraviolet light causes the entire screen to be illuminated, but eachselected spot successively struck by the beam from crystal 5 is darkenedto form a picture on the screen.

LASER MODULATOR

In a laser modulator the acousto-optical system modulates a portion ofthe output of the lasing medium. If the light is focused to less thanabout 10⁻² or 10⁻³ cm it will be modulated but not diffracted. Forgreater diameter focal spots it will be both diffracted and modulated. Alaser modulator could be used, for example, to send signals by means ofthe fluctuating laser beam intensity.

FIG. 2 shows a laser modulator of the internal configuration. In FIG. 2,lasing medium 12 produces a beam of coherent light which ismultiply-reflected between mirrors 13 and 14. Mirror 13 totally reflectsthe light and mirror 14 partially reflects it and partially transmits itas the laser output 32. Interposed between lasing medium 12 and mirror14 is a crystal 15 of Tl₃ PSe₄. (The crystal could also be positionedbetween mirror 13 and the lasing medium). To crystal 15 is affixed atransducer 16 which is electrically connected to an RF generator 17.This generator produces a radio-frequency electrical signal whichtransducer 16 coverts into an acoustic wave which moves through crystal15 diffracting light as shown at 18.

FIG. 3 shows a laser modulator of the external configuration. In FIG. 3lasing medium 19 produces a beam of coherent light which ismultiply-reflected between mirror 20, which totally reflects the beam,and mirror 21 which partially reflects the beam and partially transmitsit as laser output 22. The laser output 22 strikes crystal 23 of Tl₃PSe₄ to which is affixed transducer 24 electrically connected to RFgenerator 25. Generating a sound wave in the crystal diffracts the laseroutput causing it to strike screen 26 instead of passing throughaperture 27 in the screen.

ACOUSTIC DELAY LINE

An acoustic delay line causes an electrical signal to be delayed for thelength of time required for an acoustic signal to traverse the crystal,L/V, where L is the length of the crystal and V is the acousticvelocity. Unlike many other methods of delaying an electrical signal, anacoustic delay line preserves the original configuration of the signal.

In FIG. 4, RF generator 28 provides the electrical signal to be delayed.This signal is electrically transmitted to transducer 29 which convertsthe signal to an acoustic wave which is propagated through crystal 30 ofTl₃ PSe₄. At the other end of the crystal transducer 31 detects theacoustic wave and converts it into an electrical signal.

NON-COLLINEAR FILTER

In a non-collinear filter, the incident light 32 strikes the crystal 33at a fixed angle, φ. Only light of wavelength λ, which satisfies thecondition ##EQU1## will be defracted at the angle φ into the outputbeams 34; f is the frequency applied to the transducer 35. Light of anyother wavelength passes through the crystal undeflected. Any wavelengthof light may be selected for deflection by choosing the appropriatefrequency.

EXAMPLE

A reaction vessel was charged with 6.1311 grams thallium, 0.3097 gramsphosphorus, and 3.1584 grams selenium. The vessel was sealed undervacuum and heated at about 800° C for 1 day. It was shaken vigorously anumber a times for thorough mixing in order to produce the compound Tl₃PSe₄.

The reactant was placed in a fused quartz crystal growing tube 0.8centimeters in diameter and covered with argon at a pressure of 15inches. Using the Stockbarger technique, crystal Tl₃ PSe₄ was grown fromthe melt at a rate of 13.5 millimeters/day. The crystal was cutperpendicular to the axes to form a cube about 0.6 cm. on a side.

The refractive indices of the crystal were measured in a method ofnormal incidence, using a spectrometer with a chopped tungsten source.Wavelength selection was accomplished with narrow band interferencefilters, and detection of the deflected beam was by a photo-multiplierfor the 0.8 μ m to 1.15 μ m region, and by liquid nitrogen cooled InSbphoto-voltail detector for wavelength of the region 1.2 μ m to 5.3 μ m.The following table gives the results of the refractive indexmeasurements.

    ______________________________________                                        Refractive Indices                                                            Wavelength (μm)                                                                         n.sub.a   n.sub.b   n.sub.c                                      ______________________________________                                        0.749        3.088     3.027     3.056                                        0.825        3.028     2.967     3.000                                        1.06         2.933     2.870     2.904                                        1.15         2.916     2.857     2.883                                        1.553        2.865     2.807     2.839                                        2.66         2.834     2.773     2.808                                        3.29         2.826     2.768     2.799                                        3.365        2.825     2.765     2.798                                        3.38         2.824     2.765     2.798                                        4.35         2.820     2.760     2.795                                        4.46         2.817     2.758     2.792                                        5.26         2.815     2.756     2.791                                        ______________________________________                                         (n.sub.a is refractive index for light polarized   to the a-axis, where a,     b, and c are the crystallographically defined axes such that a = 9.270 A,     b = 11.047 A, c = 9.059 A).

The acoustic properties of the crystal were measured on a sample ofcrystal approximately 0.5 centimeters on the side. Transducers werecemented on each part of opposite faces of the crystal, and thevelocities were measured by the conventional pulse-echo method. Thelongitudinal wave velocity for propagation along the c-axis was 2.22 ×10⁵ cm/sec, and along the a- and b-axes it was 1.98 × 10⁵ cm/sec. Therewas a fast shear wave for propagation along each of the three axes, ofvelocity 1.1 × 10⁵ cm/sec. For propagation along the c-direction and thea-direction there is also a slow shear wave of velocity 5.05 × 10⁴cm/sec.

The acousto-optic figure of merit at λ = 1.15μm, relative to fusedquartz was measured for various configurations. For longitudinal waves,this relative figure of merit ranged from 500 to 1365, and for shearwaves, it reached a measured value of 1370.

We claim:
 1. A compound having the general formula Tl₃ XSe₄ where X isabout 50 to about 100% phosphorus and about 0 to about 50% arsenic.
 2. Asingle crystal of the compound of claim
 1. 3. A compound according toclaim 1 having the formula Tl₃ PSe₄.
 4. A biaxial, non-piezoelectricorthorhombic single crystal according to claim 2 having the formula Tl₃PSe₄.
 5. A single crystal according to claim 2 which is at least about.[.10⁻³ .]. .Iadd.0.05 .Iaddend.cm .[.wide.]. .Iadd.in diameter.Iaddend.and .Iadd.at least about .Iaddend.1 .[.mm.]. .Iadd.cm.Iaddend.long.
 6. A single crystal according to claim 5 which is atleast about 0.5 cm in diameter and at least about 1 cm long.
 7. A singlecrystal according to claim 5 having two parallel optical faces.